In the extraction of oil from earth boreholes, it is often required to provide some form of artificial pumping power to force the oil being extracted up the borehole to the surface where it can be collected. In such producing wells, electrically powered pumps located at the bottom of the wells are often employed. Such devices, called electrical submersible pumps (ESPs), are typically first installed after the well has been drilled and while it is being prepared to be put into production. ESPs are located at the bottom end of a long length of tubing, called the production tubing string, and are powered by electrical cables deployed from the surface. These cables are typically armored, shielded, or contained in special tubing for protection from mechanical damage during assembly and operation.
The ESPs pump oil into the production tubing string through which it flows to the surface. Failure of an ESP requires its removal from the well for replacement or repair. This is a very expensive operation, not only because of the high costs of the equipment itself and the manpower required in remote and hostile locations, but also because of the oil production lost while the well is not producing oil. Thus, reliability of the ESP is of utmost importance.
It is well known to monitor the pump current and control pump operation based upon a 24 hour "ammeter chart". Such pump operation is described in "Recommended Practice for Operation, Maintenance and Troubleshooting of Electric Submersible Pump Installations", American Petroleum Institute, Third Edition, Nov. 1, 1994. While this is one of the most common methods of monitoring ESP performance, the information is limited with respect to the physical characteristics of both the ESP and the environment in which the ESP is located. It is also known to install electrical pressure and temperature sensors with some ESPs to provide the operators on the surface with information about the pumps performance, which then allows the operators to control various parameters, such as pump speed, which can increase the life of the pump. Increasing the pump life until a scheduled maintenance, when other scheduled downhole work can be accomplished at the same time, is highly desirable, since it minimizes costs due to lost oil production.
The presently used electrical pressure and temperature sensors are limited for several reasons. The on-board electronics of such sensors must operate in a very hostile environment, which includes high temperature, high vibration and high levels of external hydrostatic pressure. Such electrical sensors also must be extremely reliable, since early failure entails a very time consuming and expensive well intervention. Electronics, with its inherent complexity, are prone to many different modes of failure. Such failures have traditionally caused a less than acceptable level of reliability when these electrical sensors are used to monitor ESPs.
There are numerous other problems associated with the transmission of electrical signals within wellbores. In general, there are many problems encountered in providing an insulated electrical conductor for transmitting electrical signals within wellbores. Such electrical conductors are extremely difficult to seal against exposure to wellbore fluids, which are at high temperatures, high pressures, and present a very corrosive environment. Such electrical conductors, once damaged by the fluids which penetrate the insulating materials around the electrical conductors, will typically short electrical signals. Additionally, electrical transmissions are subject to electrical noises in some production operations.
It is also known to use optical interferometers for the measurement of wellbore conditions, such as downhole wellbore pressures and temperatures. However, optical interferometers are typically very sensitive to temperature variations and the downhole temperature of a specific position within a wellbore will change over time, depending upon different factors such as, for example, production rates, the types of fluids produced over the life of the well, and downhole wellbore conditions. A factor affecting wellbore temperature is the injection of various treatment fluids from the surface. Such treatment fluids typically enter the wellbore at surface-ambient temperature and therefore cause substantial temperature fluctuations. Even optical interferometers designed of special material or construction are subject to inaccuracies because of the harsh borehole environment and because of the very tight tolerances in such precision equipment.
Therefore, a reliable system is needed for accurately measuring the physical properties of a harsh environment, such as a borehole. Additionally, such a system should be capable of accurately measuring physical parameters associated with a device, such as an ESP, located in the harsh environment.